Dr. Liane Smith
Wood Group Intetech
Many producing offshore oil and gas fields include a mixture of operating, suspended, and abandoned wells. One continuing challenge for the operators is to distinguish between those wells that have been suspended (i.e. for workovers) or abandoned safely, and those that are not correctly treated.
Suspended wells can encounter a variety of problems, and wells that have been permanently plugged and abandoned (P&A'd) can start to leak. There are numerous possible causes. Over time thermal and pressure cycling can generate micro annuli or cracks in the cement seal or other formations above the reservoir, while leak paths can develop from the inside of the wellbore outwards, or in the plugs themselves if they have not been positioned or tested properly.
Nevertheless, provided there is some structure left at the surface, there is scope for monitoring the condition of an abandoned well – even if it means using a single pressure gauge to ensure a reading of nil above a plug or wellhead. Best practice is to continue to monitor abandoned wells for a number of years, using a well integrity management system.
Well integrity management systems combine well operating and production data within a framework for decision-making, management processes and organizational structure. They also enable operators to maintain a full history of their wells. Analysis of historical data not only provides critical intelligence for managing abandoned wells safely, but helps operators get more from their assets.
Levels of suspension
There are many reasons why a producing well might be shut-in. Often shut-ins are necessary to perform routine maintenance of surface facilities, equipment or pipelines, rather than the well itself. The operator might also be experiencing lower than expected rates of recovery.
Meanwhile, integrity issues can arise from sources such as scaling, corrosion, failed well barrier equipment, or sustained annulus pressure. The latter is the number one killer of wells and can lead to an external leak or, at worst, a blowout. Even if no leakage occurs, when pressure within the well rises above its safe operating envelope, the resultant failure of well barrier equipment can be too risky or costly to repair and therefore the well must be abandoned.
If a well is being shut-in to discontinue flow for a short period, this is achieved by simply closing the surface safety valve (typically, the upper master valve). If the well has to remain suspended for a long period of time, a heavy liquid such as brine is pumped in to prevent fluid flow. This can then be removed later to re-start flow.
Should permanent abandonment be required, the well is plugged by setting mechanical or cement plugs in the wellbore at specific intervals to prevent fluid flow. The P&A process normally requires a workover rig and pumping of cement into the well to restore the natural integrity of the formation penetrated.
Properly plugged wells can save the operator substantial sums through avoidance of lost production. If a well is not properly abandoned, it may provide pathways for brines, hydrocarbons, or other fluids to migrate to surface, leading to the technically demanding and costly task of re-entering the abandoned well to plug a leak.
Click to Enlarge
|Overview of well types and states for different reservoirs with integrity status color coded. (Image courtesy Wood Group Intetech)|
In regions such as theNorth Sea and Asia/Pacific, the growing number of aging assets has meant operators must adopt more robust approaches to managing well abandonment. Britain's Department for Energy and Climate Change (DECC) estimates that as of 2012, there were 740 suspended wells on the UK continental shelf with around 5,000 wells to be decommissioned.
Oil and Gas UK has guidelines to steer operators on the considerations that need to be taken when suspending operations in a well for a limited time, re-entering a well safely, and abandoning a well. The guidelines provide minimum criteria for ensuring full and adequate isolation of formation fluids both within the wellbore and from surface or seabed. The Well Suspension and Abandonment workgroup of the association's Well Life Cycle Practices Forum has also produced guidelines on qualification of materials for the suspension and abandonment of wells.
In the US, regulatory standards require specific provisions for plugging and documenting oil and natural gas wells before they are abandoned. P&A regulations vary to some degree among states, but all prescribe the depth intervals that must be cemented, as well as the materials allowable in plugging practices.
Despite a consensus that the processes and regulation relating to abandonment need to be vastly improved, wells continue to be plugged at the lowest possible cost and in accordance with the minimum requirements mandated by regulators. This is because P&A work is capital-intensive and provides no return on investment for operators.
In the US and Europe, the OSPAR Convention – the current legal instrument guiding international cooperation on the protection of the marine environment – mandates that an abandoned well remains the responsibility of the owner. According to a report issued last year by the UK's Royal Academy of Engineering on decommissioning in the North Sea, should ownership of an asset be transferred during the field's lifespan, liability transfers to the new owner in most cases. However, there are cases where it remains with the seller. For example, should a new owner default, liability reverts to the original license holder. As such, the report notes that there is a generally accepted understanding within the industry that, "if you put it there, you take it away." Operators must therefore take the utmost care to protect the environment at all stages of the P&A process to avoid potential remediation or litigation costs should issues with an abandoned well arise at a later date.
It is worth noting that recent high profile incidents in theGulf of Mexico and the North Sea occurred during operations related to well abandonment. These events serve to highlight the importance of adopting an asset management strategy that ensures the continuous integrity of wells from design and construction, to operation, and finally abandonment.
Getting ahead of the curve
Scaling, corrosion, and failed well barrier equipment are all common issues that call for vigilance to minimize the risk of leakage. The fact is that thermodynamics always catches up with you: When there is a piece of unprotected steel in the ground it will corrode. Any new engineering system will have some failures due to human error or material defects. There will also be a steady rate of failures that can be corrected routinely by maintenance and repair, before corrosion or fatigue damage eventually sets in.
However, many oil and gas operators find that the level of well "infant mortality" is far too high – wells that come into service and quickly develop various integrity issues. Equipment leak testing and annulus pressure monitoring also identify a high frequency of failures with the associated maintenance demands that this brings. As a consequence, many wells are terminated prematurely before the end of their cost-effective life. Tools that allow well integrity to be controlled at every step tackle all these problems and result in wells achieving their design objectives. An advanced well integrity system enables operators to consolidate all of their well data and manage the full lifecycle of their wells.
Using web-based well integrity management software with smart functionality make it possible to acquire information from multiple sources – including third-party databases and legacy systems – and to display it in a single management dashboard. This provides an instant view of current well integrity status as well as historical key performance indicators (KPIs) at a well, field, or enterprise level.
Operators receive email warnings of wells where conditions indicate that well barriers are failing and there is a risk of a leak. The software also performs added-value analysis of the data collected to identify developing problems. This allows operators to track wells that are at risk of failing and to take preventative action to keep them safely in production. In addition, they can ensure a safe and effective P&A or well suspension at the first attempt because the well condition is known in detail.
If a well is suspended properly, and the operator has a full history of the materials used, equipment installed, and the damage the well may have sustained over its life, it may also be possible to recover or re-design the well. Thanks to advances in drilling and well design, temporarily abandoned wells could prove to be extremely valuable assets. Crucially, using historical well integrity data to improve front-end processes and equipment selected based on past performance and lessons learned can drive down operating costs, optimize maintenance, reduce workovers, and increase profitability of well operations in the long term.
Editor's note: Intetech, which was recently acquired by Wood Group, is now known as Wood Group Intetech, part of Wood Group Kenny.